Some types of floating vessels, such as drill ships, floating platforms, drilling semi-submersibles, floating production storage and off-loading vessels (FPSOs) and the like use electric propulsion systems in which propellers are driven by electrical motors. Examples include azimuth thrusters of a vessel, which include an electric motor arranged in a pot. The electric motor is directly connected to the propeller. Electricity may be produced on board of such vessels, for example, by one or more generators coupled to a diesel engine or gas turbine, and transmitted by an onboard power grid to the electric motor for driving the thruster. Such vessel may include further electric drives that require electrical energy for operation, including anchor winch drives, drives of drilling applications, drives of drawworks, and the like. Such vessels may also include electric motors operating auxiliaries.
Marine vessels employing such types of electric drives may include an onboard power plant that produces the required electrical energy, for example, by the above mentioned diesel engine or gas turbine coupled to a generator. Such arrangements provide the main power supply for the vessel. If the power plant experiences a blackout, or if there is a fault in the onboard power grid or in a component coupled thereto, the vessel will lose its main power supply.
The electric motors (e.g., of the thrusters) may be driven by a variable frequency drive for enabling speed control. If a blackout of the main power supply occurs, these frequency converters map trip on under-voltage and may be out of service for the duration of the blackout. After the onboard power plant resumes operation, the variable frequency drives require a certain startup time to come back online and to start supplying electric power to the electric motors. Restarting the variable frequency drive may also include the starting of all auxiliary devices, such as cooling pumps, fans, lubrication pumps and so on. The duration for restarting a variable frequency drive may be significant.
Although the power plants of such vessels may be designed to restore electric power as fast as possible, and the electric drives are designed to restart as fast as possible, there may be a significant delay before the electric drives may continue to operate. Long delays are undesirable, for example, for marine vessels having electric propulsion systems, as inoperability of the thruster drives may lead to a loss of position and maneuverability. Vessels operating dynamic positioning systems of higher classifications, such as equipment class 2 (DP2, e.g., DYNPOS-AUTR) or equipment class 3 (DP3, e.g., DYNPOS-AUTRO) and the like (see IMO MSC/Circ.645, “Guidelines for vessels with dynamic positioning systems”), thus have a certain redundancy in the power generation systems. The power generation system may, for example, be split into several separate and independent subsystems. For DYNPOS-AUTRO classification or DPS3-classification, the main bus bar is, for example, split into two or more different sections, each being connected to a generator. Bus tie breakers provided between the sections of the main power distribution bus may be open during class 3 operation so that one section is not affected by a blackout in another section.
This effort may be made since the time that the thrusters require for coming back online after a blackout is too long. One of the reasons for this is the startup time required for the frequency converters. Operating the vessel with split sections of the power system makes operation rather ineffective and leads to a higher fuel consumption and thus increased operating costs.